Repeated press-molding fine glass optical elements needs dies mechanically strong enough not to lose their shape during press-mold processing. Such dies should be stable to high temperatures, resistant to oxidation, and inert with respect to glass. Further, the dies must be excellent in machinability so as to be machined precisely and easily.
Conventional press-molding dies include, for example, a die of mixed materials including titanium carbide (TiC) and a metal, as disclosed in Japanese Unexamined Patent Publication SHO 59-121126, and a die composed of cemented carbides and a precious metal thin layer formed thereon, as disclosed in Japanese Unexamined Patent Publication SHO 62-96331.
These conventional dies, however, could not satisfy all the conditions. Although the die made of TiC and a metal is hard and mechanically strong, it is very difficult to machine the dies precisely. In addition, the die has the problem of reacting with components of glass optical elements, such as lead (Pb) or alkaline metals.
On the other hand, dies made of cemented carbides coated with a thin layer of a precious metal are superior in mechanical strength and easy to process. They hardly react with glass in an inert gas atmosphere, such as nitrogen gas, up to about 600.degree. C. during repeated press-molding. However, at 650.degree. C. or more, the surface of the dies changes in color, and becomes cloudy after about 100 times of press-moldings. Thus, the dies cannot withstand further use. At 650.degree. C. or more, the dies show no satisfactory wetting to and releasing from glass.
To improve the press-molding dies, a die composed of cemented carbides coated with a nickel (Ni)-precious metal alloy thin layer is proposed in Japanese Unexamined Patent Publication SHO 64-61327. The proposed die still has a problem of suffering grain growth after 1000 times of press-moldings at 650.degree. C. or more, and becoming rough. Thus, the die can not repeatedly press-mold higher-melting glass.
Therefore, higher-melting glass optical elements cannot be mass-produced using the conventional press-molding dies.